Most major musculoskeletal impairments affect the mobile segments of the body, including the synovial (e.g. knees, hips and shoulder) and cartilaginous (e.g. spine, neck) joints. The mobility of these joints can increase or decrease with damage or disease: Osteoarthritis often results in loss of joint motion while ligament or capsular injury of the knee, hip or shoulder results in greater joint motion (laxity). In either case, characterizing joint motion is an important part of the diagnosis and an even more important part of the outcome assessment after treatment.
Unfortunately, accurate assessment of joint motion is challenging. The joints lay beneath layers of skin, muscle and fat that make direct observation or quantification of joint motion difficult. Further complication arises because motion of many joints is significantly influenced by muscle contraction and joint load. Thus, joint motion may differ dramatically depending upon the activity being performed or the muscles activated. These challenges are highlighted in the American Academy of Orthopaedic Surgeons' 2003 Unified Research Agenda which states the following unfilled needs:                1. Study of joint kinematics around the hip, ankle, elbow, shoulder, wrist, and knee in normal, arthritic, and reconstructed states with the development of high-speed computational methodologies to stimulate natural and artificial joint performance.        2. Exploration of the role of novel imaging technologies on joint arthroplasty, including RSA, DEXA, surgical navigation, minimally invasive and robotic surgery.        3. Improving the ability to diagnose spinal disorders, including the ability to localize the source of pain, evaluate motion segment instability, and evaluate the role of muscles and connective tissue on back pain.        4. Study of pathomechanics of joint injury focusing on prevention and the development of more effective protective devices for particular sports and jobs where risks of physical impairment exist.        
Each of these items described above requires the ability to measure joint motion or kinematics. This problem has been addressed with a variety of measurement techniques. Methods utilized clinically include serial imaging (radiographs, CT, or MR) with joints stressed in different positions and instrumented laxity testing devices. These techniques permit detailed measurements of joint motion, but do not permit assessment with normal dynamic joint loads or muscle loading.
Motion capture (MoCap) technology also has been used in clinical and research contexts and does permit motion measurement during normal dynamic motion. However, MoCap relies on markers placed at the skin surface, which cannot provide accurate bone motion measures when there is significant muscle/skin/fat overlying the joint, such as the knee, hip, shoulder, or spine.
One solution providing detailed measurement of bone or implant motion during dynamic activity is fluoroscopy, which is essentially x-ray video. In 1991, the present inventor reported the first use of fluoroscopic imaging to quantify two-dimensional (2D) knee replacement motions (Banks S A, Riley P O, Spector C, Hodge W A: In Vivo Bearing Motion with Meniscal Bearing TKR. Orthop Trans 15(2): 544, 1991) and completed a fully three-dimensional (3D) measurement technique based on shape registration in 1992. The present inventor reported further elaborations of this measurement approach and its application to a variety of different devices during a range of activities. This work received international awards, demonstrating the potential for significant worldwide clinical impact. However, the utility of this measurement approach is limited by factors including activity restrictions, anatomy limitations, exposure to ionizing radiation and bone model generation.
Regarding activity restrictions, patients suffering from rupture of the anterior cruciate ligament of the knee can have quite different clinical outcomes depending upon how they recruit their muscles as dynamic joint stabilizers. Similarly, the motions of knee replacements differ significantly depending upon the activity. These findings underscore the need to observe motions during normal loading conditions when deciding on surgical interventions or assessing device design/performance relationships. The challenge is finding clinically important activities where the joint remains in a small volume of space suitable for fluoroscopic observation generally within a 23-30 cm field of view. This challenge has been addressed for the knee by using modified stair-climbing and gait activities where a combination of activity modification and multiple positions of the fluoroscope are used to obtain the required images. However, it has not been possible to observe knee motions during normal ground-level steady walking, stair descent, or chair sit/stand. Furthermore, available imaging systems do not permit observations centered less than ˜100 cm from the ground, requiring platforms to elevate the subject into the field of view. This means more equipment is required, the subject is elevated into a visually uncomfortable position, and accessory sensors like floor-mounted force plates for measuring ground reaction forces are not easily used.
Regarding anatomy limitations, “lift with your legs” is a simple, sensible admonition to avoid low back injury. However, it precludes fluoroscopic observation of the low back during relevant lifting activities due to a small field of view. Similarly, it is impossible to obtain a transverse view of the patella, a so-called skyline view, during a chair-rise or bicycle activity without an imaging system that can dynamically align the view to the anatomy of interest. Similar arguments apply to the shoulder, foot & ankle and hip where biomechanically interesting or clinically important observations are not possible because of the limited field of view or immobility of existing radiographic imaging systems.
Regarding exposure to ionizing radiation, fluoroscopy involves exposure to ionizing radiation. Although this often is justified by the clinical need or importance of the information so obtained, exposure must be minimized to the extent possible. Current systems rely on human operators to initiate and terminate exposures. Because the operator has only qualitative feedback on the timing and alignment of the exposure, the patient receives greater than the minimum exposure required for the well aligned & timed diagnostic image sequence.
Regarding bone model generation, the model-based image measurement approach assumes a digital model of the implant or bone surface is available for registration with the radiographic image sequence. Obtaining models for manufactured objects is not difficult. However, it is inconvenient, expensive and time-consuming to obtain separate tomographic scans of the patient or study subject to create the bone-models required for dynamic radiographic motion analysis.